Powder metallurgic manufacturing process

ABSTRACT

An improvement in a powder metallurgic manufacturing process where metal powder is compressed and heated in a mould and heat is produced by conducting electric current through the mould and the metal powder as well as through at least one electrically resistive fireproof body in actual contact with the mould where heat produced in the body is transferred to the mould and the metal powder. The improvement comprises maintaining electric tension between 2 and 200 volts; the current efficiency per unit volume of the mould between 20 and 200 W/cm 3  ; and the resistivity of the electrically resistive fireproof body between 0.03 and 100 ohm × cm.

This invention relates to a powder metallurgic process by means of whichmetal powder or metallic powder mixture is compacted and heated in amould. This metallic powder mixture consists of metal powders and suchsubstances as diamond, boron nitride and other abrasives. At thebeginning of the process the mould and the metallic powder mixture arecold. The heat is produced by conducting electric current through themould and the metallic powder mixture.

The hot-pressing, i.e. simultaneous compacting and heating, has theadvantage of great freedom for choosing the compact composition andtherefore the possibility for getting special characteristics in thecompact. The sintering time is short and the temperature is relativelylow, which properties can be most important when the metallic powdermixture consists of such evaporable or reactive substances as zinc,graphite, diamond or similar materials.

The hot-pressing process has also disadvantages, e.g. expense due to themould and difficulties in transferring heat to the compact. The mouldsare of graphite, steel or hard metal. Only in a few cases can a graphitemould be heated simply by conducting current through it. The current inthat kind of short circuit is very high (6000 . . . 10.000 A) andreciprocally the tension is very low (0.3 . . . 1 V).

Steel and hard metal moulds, where the resistivity is about 25 timeslower than in graphite, should be heated to get the same efficiency with5 times higher current and 5 times lower tension. It is so difficult toobtain these conditions that generally a high frequency electric heateror a separate furnace is chosen. Graphite moulds are often heated in thesame way.

This invention aims at a simplification of the use of moulds inhot-pressing. The characteristics of the invention are to be found inthe patent claims.

This invention is based on the insight that it is possible to make ofthe mixture of graphite and fireproof material such parts of mould whichhave controlled electric resistance, and which thus with a safe tensionand a moderate current develop the required heat. Other fireproofresistive materials such as silicon carbide and Si--C--O-compounds, actlike graphite, but their manufacturing price make them lessadvantageous. The hydraulically or chemically binding concretes aresuitable for the basic material of fireproof resistive parts, theircement or binding material being alumina cement, Portland Cement,water-glass, phosphoric acid, magnesia or doloma cement. The hardenedfireproof resistive concrete is made from these cements and graphitelike corresponding fireproof concrete.

The resistivity of graphite concrete is about 0.001 × c⁻⁵ ohm × cm,where c = concentration of graphite.

Graphite concrete, such as corresponding fireproof concrete, must bedried after hardening to prevent the explosion caused by steam pressureduring the heating phase. The temperature rise in hot-pressuring is veryfast and therefore a careful drying as well as the evaporating ofchemically bound water is necessary. In the heating system there arethus two very different resistive composites. The mould, the compactedpowder and the punch are very low-resistive and with few possibilitiesof choice. Often the punch, and especially a thin-walled punch, becomestoo hot with the current necessary for heating the mould and compact.

The other resistive composite, the above mentioned parts of moulds,which have controlled electric resistance, can be freely chosenhigh-resistive and thus the following advantages are obtained:

A moderate and remarkably lower current than mentioned in theintroduction and a safe tension, like 2 . . . 200 V, or moreconveniently 5 . . . 20 V, can be used.

The heat, fully controlled, is generated in the right places

Simultaneously with the compacting can be obtained the fixing of thecompact to the punch or to the mould, according to requirements

The using of steel and hard metal moulds is self explanatory

The utilization of this invention is favoured by the wide distributionof the power source needed. A safe tension of 20 . . . 50 volts andefficiency enough can be obtained from many normal welding apparatus. Itis easy to calculate and to test under these conditions the rightresistivity for the graphite concrete.

In the sectional FIGS. 1 and 2 are shown schematically the typicalsolutions which are different from each other.

In both figures the hot pressing is made between two electrodes 1, ofwhich the current of safe tension is obtained. FIG. 1 shows themanufacturing of a grinding wheel, where the metal punch 3 is strong andshort and does not buckle even when hot. The circuit here islow-resistive, and so the solution without extra resistors 2 wouldrequire a very high current. In this solution two resistors 2 make theheat conduction even and the heated parts do not become weak.

FIG. 2 shows the manufacturing of a drill core bit. A long and thinwalltube 3 acting in this case as punch might easily buckle and very littleadditional heat can be developed in it. One heating resistor 2 must besufficient and it might be necessary to cool tube 3 or it must bestrengthened during the hot pressing so that it has much allowance formachining.

The resistive composites used in this method are plates of eventhickness, and by a plate thickness of merely 5 to 10 mm an even heatdevelopment can be achieved and the appearance of the electric arc canbe prevented. In practice, the resistive plates 2 and the compact 4 aresubjected to the same force which is rather high, but the resistivecomposite must not break from that. This kind of strong resistive platecan be made of the mixture of graphite powder, alumina cement andasbestos. After wetting and pressing by a pressure of 100 . . . 200 bar,this mixture is left to harden and finally to be dewatered.

This invention thus relates to such a powder metallurgic hot pressingmethod where the heating of pressure mould 5 and the metallic powdermixture 4 therein is produced by conducting the electric current throughthem as well as through one or several resistive graphite containingbodies 2 in contact with mould 5, in which resistive bodies the heatthus produced is transferred into mould 5 and into the metallic powdermixture 4. The metallic powder mixture in the above mentioned examplesconsists of cobalt and/or iron and natural and/or synthetic diamond. Theinvention is naturally not limited to these embodiments but can bemodified in many ways within the scope of claims.

I claim:
 1. In a powder metallurgical manufacturing process, where metalpowder or a metal powder mixture is compressed and heated in a mould andthe mould and the metal powder are cold at the beginning of the processand where heat is produced by conducting electric current through themould and the metal powder as well as through at least one electricallyresistive fireproof body in outer contact with the mould, in which bodythe heat thus produced is transferred into the mould and into the metalpowder the improvement wherein:(a) the electric tension is between 2 and200 V; (b) the current efficiency per unit volume of the mould isbetween 20 and 200 W/cm³ ; and (c) the resistivity of the electricallyresistive fireproof body is between 0.03 and 100 ohm × cm.
 2. In aprocess as claimed in claim 1, wherein:(a) the electric tension isbetween 5 and 50 V; (b) the current efficiency per unit volume of themould is between 50 and 100 W/cm³ ; and (c) the resistivity of theelectrically resistive fireproof body is between 0.1 and 10 ohm × cm. 3.In a process as claimed in claim 1, characterized in that the resistivefireproof body consists of graphite and concrete.
 4. In a process asclaimed in claim 3, wherein the graphite is present in an amount in therange of from 15% to 40% and the concrete is present in an amount in therange of from 60% to 85%, said percentages being based upon the totalweight of said body.